1. Field of the Invention
The present invention relates to a structure of a copper conductor and a method of forming the same. More specifically, the present invention relates to a structure of a copper film formed on the surface of a substrate such as ceramics, metal, resin or the like, with a strengthened adhesion thereof to the surface of the substrate, and a method of forming the same.
2. Description of the Prior Art
A copper conductor attained in accordance with the present invention can be used as an electrode or a conductive device in various kinds of electronic circuit components. One example of a copper conductor of interest to the present invention is an electrode or electrodes of ceramic capacitors. The electrode of a ceramic capacitor was generally formed using silver of high conductivity. Such a silver electrode was formed by coating a silver paste on a ceramic body and by baking the same. However, with the recent rise of the cost of silver material, the ratio of the cost of such a silver electrode to the total cost of a ceramic capacitor increased. Thus, employment of such silver electrode became a cause of the rise of the total cost of ceramic capacitors.
In such situation, attention was given to development of an inexpensive electrode. As one approach, various studies were made as to methods for forming a metallic film, such as by electroless plating process, vacuum evaporation process, sputtering process, ion plating process, and the like. As another approach, study was made as to employment of inexpensive metal in substitution for silver as an electrode.
The first approach employed was a nickel plated electrode formed by means of an electroless plating process. A nickel plated electrode was successful to some extend as an inexpensive electrode as a substituttion for a silver electrode. However, it was observed that employment of a nickel plated electrode as an electrode of ceramic capacitors involves the following problems. More specifically, the resistivity of a nickel electrode per se is 7.24.times.10.sup.-6 .OMEGA..cm, which is higher than that of silver which is 1.62.times.10.sup.-6 .OMEGA..cm. Accordingly, a problem is caused in that a frequency characteristc is degraded in the high frequency region. Another problem is that solderability of a nickel plated electrode is poor. Furthermore, another approach was attempted where the whole surface is coated with a solder layer in order to decrease the resistivity of the nickel plated electrode. However, in coating the whole surface of the electrode with a solder layer, a large amount of flux which is active must be used. Accordingly, it is necessary to cleanse the electrode to remove unnecessary flux after soldering. Furthermore, although in soldering the electrode portion is processed by dipping the same in a solder tub, for example, such process causes stress in the ceramics, so that a crack is liable to be caused in the ceramics.
Development of an inexpensive electrode that can be substituted for the above described nickel electrode was desired. Therefore, a new attempt was made with a copper electrode. However, a large obstacle was involved also in such copper electrode. More specifically, adhesion of a copper film constituting an electrode to a ceramic substrate is extremely poor. Copper has also been a typical example of metal having an adhesion to a ceramic substrate as poor as that of gold. However, in view of the fact that copper has a high conductivity, it can be said that copper is a preferred material for constituting an electrode or a conductive device, apart from the above described problem of a poor adhesion.
As a means for compensating for the above described problem involved in the adhesion of a copper film, conventionally an approach to be set forth in the following was employed. In the case where a copper film is formed by a vacuum evaporation process, for example, a so-called multilayer evaporation was employed in which a metal of a small atomic radius or a metal easily oxidized such as chromium, aluminium or the like is first evaporated on the surface of a ceramic substrate and then copper is evaporated thereon, whereby a copper film of a strong adhesion is attained. More specifically, through interposition of a metal of a small atomic radius or oxidized metal between a copper film and a ceramic substrate, a copper film stably adhered to a ceramic substrate was obtained. However, such approach involves an increased number of processing steps and can not be said to be a simple method.
Furthermore, in the case where a copper film is formed by an electroless plating process, a ceramic substrate is first dipped in a water solution of fluoric acid, for example, to apply an etching process thereto, so that the surface of the ceramic substrate is made uneven and then a copper film was formed. More specificaly, the adhesion of the copper film to the ceramic body was increased by an increased mechanical contacting force. However, when a ceramic substrate such as a dielectric, insulating, semiconductive or resistive meterial is subjected to an etchant such as fluoric acid, the surface of the ceramic substrate is corroded and as a result not only the function of the ceramic substrate is changed but also the same is degraded. Thus, such means is not preferred.
Meanwhile, the problem of a poor adhesion of a copper film is involved not only in case of formation thereof on a ceramic substrate but also formation thereof on metal, resin or the like.